1. Field of the Invention
The present invention relates generally to infrared imaging devices and, more particularly, to an improved infrared detector.
2. Related Art
Imaging devices that detect various regions of the electromagnetic spectrum are used in a variety of commercial and military applications. One type of imaging device uses a focal plane array (FPA) to detect infrared radiation. An FPA, for example, may be formed from an array of infrared detectors, with each infrared detector functioning as a pixel to produce a two-dimensional image. The change in signal level output of each infrared detector due to received incident infrared radiation is translated into a time-multiplexed electrical signal by circuitry known as the read out integrated circuit (ROIC). The combination of the ROIC and the infrared detector array is commonly known as a FPA or infrared FPA. FPAs (e.g., microbolometer FPAs) are described in further detail in U.S. Pat. Nos. 5,756,999 and 6,028,309, which are herein incorporated by reference in their entirety.
FIG. 1 shows a cross-sectional view of one exemplary type of a conventional infrared FPA 100, which includes a substrate 102 made of indium phosphide (InP), an active detection layer 104 made of indium gallium arsenide (InGaAs), and a substrate 108 made of silicon (Si). Active detection layer 104 serves to detect infrared radiation 110 and is electrically coupled to substrate 108 through bump-bonds 106 (e.g., indium bump interconnects). The ROIC is formed on substrate 108 to read out the data collected by active detection layer 104.
Substrate 102 is required to grow the InGaAs epitaxial layer that forms active detection layer 104. For example, In(x)Ga(1-x)As is typically selected to be In(0.53)Ga(0.47)As, because its lattice constant matches the lattice constant of the InP material of substrate 102. After the InGaAs epitaxial layer is formed, the InP material of substrate 102 generally functions as a transparent window that passes the infrared radiation 110 that is to be detected by active detection layer 104. Active detection layer 104 of In(0.53)Ga(0.47)As and substrate 102 of InP, however, result in infrared FPA 100 having a limited spectral range or spectral sensitivity between 0.9-1.7 μm. As a result, there is a need for an improved infrared detector array that provides a wider spectral range.